1. Field of the Invention
The invention relates in general to a method of controlling an output power of a laser diode in an optical drive, and more particularly to a method of determining the time of adjusting writing strategy of an optical drive.
2. Description of the Related Art
A laser diode of an optical drive is disposed in an optical pickup head to serve as a light source for reading/writing an optical disk. When the optical drive is writing, the laser diode has to continuously output a higher and stabler power. FIG. 1 is a schematic illustration showing a closed-loop control circuit for an output power of a laser diode in an optical drive. First, a DAC (Digital to Analog Converter) 10 receives a digital control signal 12 and converts the digital control signal 12 into an analog control signal 14. An error signal 16, which is obtained by subtracting an analog feedback signal 42 from the analog control signal 14, is inputted to a compensator 20. The compensator 20 can generate an output signal 22, which is outputted to an amplifier 25, according to the error signal 16. Thus, the amplifier 25 can convert the output signal 22 into a driving current 28, which flows to a laser diode (LD) 30. After the LD 30 emits a laser beam 32 according to the driving current 28 and a FMD (Front Monitor Diode) 40 has received the laser beam 32, the FMD 40 outputs the feedback signal 42.
FIG. 2 is a schematic illustration showing the writing strategy for a rewritable-disk in the optical drive. In general, owing to the material property of the recording layer of the rewritable-disk, the EFM (Eight to Fourteen Modulation) signal has to be converted into a pulse-type high-frequency (HF) signal (write power Pw, base power Pb, and erase power Pe), and the HF signal can be used to drive the laser diode to output the pulse-type laser beam. Therefore, the recording information such as pits and lands can be formed on the tracks of the recording layer. For example, the EFM signal is a signal ranging from 3T to 11T according to the specification of the rewritable optical disk. When the EFM signal is 3T, the HF signal has two pulses.
Typically, a temperature sensing element, such as a thermister, is usually disposed in the optical pickup head of the optical drive such that the ambient temperature around the optical pickup head can be monitored momentarily. When the optical drive is executing a writing control, the ambient temperature around the optical pickup head greatly influences the written quality of the optical disk. That is, different writing effects occur when the same HF signal is used to drive the laser diode under the environment of different ambient temperatures. Hence, when the temperature in the optical drive changes, the optical pickup head has to adjust the writing strategy of the optical drive.
The writing strategy adjusting operation enables the optical drive to change the pulse width of the HF signal so as to drive the laser diode and write the optical disk after the temperature in the optical drive is changed. Otherwise, when the temperature in the optical drive is changed but the HF signal for driving the laser diode is not changed correspondingly, the written quality of the optical disk is deteriorated, or the writing operation may fail. In general, as the ambient temperature of the optical drive rises, the pulse width or the write power of the HF signal has to be decreased so that better information may be recorded in the optical disk.
In order to reduce the manufacturing cost of the optical drive, however, no temperature sensing element is disposed in the optical pickup heads of some optical drives. Therefore, it is very important to properly determine the time of adjusting the writing strategy of the optical drive in order to prevent the written quality of the optical disk from being deteriorated or prevent the writing operation of the optical drive from failing when the optical disk is writing.